11.4 gethostbyaddr Function

The function gethostbyaddr takes a binary IPv4 address and tries to find the hostname corresponding to that address. This is the reverse of gethostbyname.

#include <netdb.h>
struct hostent *gethostbyaddr (const char *addr, socklen_t len, int family);
Returns: non-null pointer if OK, NULL on error with h_errno set

This function returns a pointer to the same hostent structure that we described with gethostbyname. The field of interest in this structure is normally h_name, the canonical hostname.

The addr argument is not a char*, but is really a pointer to an in_addr structure containing the IPv4 address. len is the size of this structure: 4 for an IPv4 address. The family argument is AF_INET.

In terms of the DNS, gethostbyaddr queries a name server for a PTR record in the in-addr.arpa domain.